The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Radio-frequency identification (RFID) provides for communication via radio waves to transfer stored data from a tag to a reader. A reader is configured to transmit radio frequency waves (RF) to a tag to collect data from the tag, and the tag may be configured to transfer stored data to the reader by modulating the transmitted RF in a manor detectable and understood by the reader. A tag configured to modulate transmitted RF is typically powered by the transmitted RF and is typically not self-powered, such as by a battery or the like. Alternatively, a tag may be configured to independently transmit RF back to the reader after the tag detects a known RF signal transmitted by a reader if the tag is self-powered. Because RFID uses radio waves for communication, the tag and the reader do not have to contact one another. This non-contact communication of RFID contrasts the typical communication between a card with a magnetic stripe (mag stripe) and a mag stripe reader where the mag stripe of the card is run through the mag stripe reader so that a pick-up coil or the like in the mag stripe reader can read information encoded in the mag stripe. This non-contact communication of RFID also contrasts with the typical communication between a smart card and a smart card reader where the electrical contacts on the smart card make electrical contact with corresponding electrical contacts on the smart card reader so that the smart card reader can read or write information on the smart card.
Due to the non-contact communication of RFID, numerous applications have been developed for this technology. The applications often center around a reader being able to identify a tag as being unique by collecting unique information stored in the tag, for example, in a semiconductor memory. For example, tags are often used in employee badges, which are provided to employees of a company so that an employee may hold her employ badge up to a reader to gain entry to the company's buildings. People without a tag encoded with the particular unique information for accessing the company's building will generally be denied entry to the company's buildings by the reader, which would not collect the particular unique information for building entry. The foregoing example of employees gaining secure entry to a building is one example of a more general application of access control.
In addition to access control, RFID is also used for the identification of products, automobiles, animals, and are used for electronic passports and electronic ticketing. For example, via the detection of products on store's shelves where the products include tags, a relatively quick and accurate inventory of products may be taken.
Additional applications of RFID include banking applications, applications for making purchases, reading tags embedded in smart posters and the like. For these additional applications a mobile device (e.g., a smart phone, a personal digital assistant, an iPod like device, etc.) may include a reader and a tag. For example, a mobile device that includes a reader may be configured to read a tag in a smart poster and display a website associated with the smart poster. Alternatively, a mobile device's tag may be read by a reader external to the mobile device for a purchase that the user of the mobile device is making and the reader may affect a charge to the user's bank account for the purchase.
Near field communication (NFC) is one particular type of RFID where the interaction distances between a reader and a tag are relatively short, for example, up to about 3 centimeters. The relatively short communication lengths of NFC provide one measure of security from a tag being read surreptitiously by a fraudulent user. Therefore, NFC is particularly well suited for use for applications requiring high levels of security, such as the exchange of personal health information, pairing devices (e.g., pairing a headset to a mobile device, pairing two mobile devices, etc.), banking transactions, consumer purchases, or other monetary transactions.
One goal of the standardization organizations that set the standards for RFID and NFC is that a reader and a tag in a mobile device share an antenna. One reason for this goal is to minimize the number of antennas in a mobile device so that interference between the often numerous antennas in mobile devices may be minimized. For example, a typical smart phone may include a radio frequency (RF) antenna for mobile telephone type communications (e.g., voice, messaging, etc.), a WiFi antenna for Internet type communications, a BlueTooth antenna, and a GPS antenna for GPS type communications. Providing one antenna for a reader and a tag limits the design considerations for reducing antenna interference. Further, providing that a reader and a tag share an antenna in a mobile device limits the number of electronic components included in the mobile device and thereby limits the cost of manufacturing the mobile device.
A number of design objectives and concerns exist for a reader and a tag sharing an antenna in a mobile device. For example, one design provides a single integrated circuit (the single integrated circuit is referred to herein as the “IC”) configured to operate as a reader and as a tag in combination with the antenna. Some designs for the IC try to have as few pins as possible that connect with an antenna interface between the IC and the antenna. The IC may provide for frequency tuning to support various antenna sizes and to tune the antenna to avoid interference with other RF signals. Some mobile device manufacturers specify the use of a relatively large antenna that is in the periphery of the mobile device, whereas other mobile device manufacturers specify the use of a relatively smaller antenna. The IC may also provide for relatively high power dissipation for an antenna collecting relatively large RF energy from a reader emitting a relatively strong RF field. Typically, the dissipation of relatively high power requires a relatively large semiconductor substrate to provide for relatively high power dissipation. As is typical of numerous circuits in mobile devices, mobile device manufacturers make attempts to reduce the foot prints of RFID circuits in mobile devices.
Other designs for a reader and a tag sharing an antenna may include providing that the IC and the antenna have a relatively high transmission drive capability to drive a relatively high power RF signal for reader operation with a weak tag operating according to relatively older ISO operating standards. The IC and antenna may be configured to have a relatively sensitive receiver sensitivity so the IC and antenna operating as a tag are sensitive to a relatively weak reader and/or provide for relatively larger operating distance from the reader (e.g., up to about 2 centimeters). In a reader mode, the drive current of an antenna may be set to a relatively small value if a tag is not detected by the reader, and may be set to a higher value if a tag is detected by the reader. In some designs, the IC and the antenna operating as a tag sufficiently load modulates a received signal from the reader so that the reader may detect the tag, for example at relatively large distances. The IC and the antenna operating as a reader may be sufficiently sensitive to load modulation of the received signal by a tag so that the reader may acquire data from the tag at relatively large distances. It is difficult to achieve these types of designs in a single RFID circuit design.
As described briefly above, in an RFID circuit the set of external components that lie between the IC and the antenna are often referred to as the antenna interface. The antenna interface tends to have a strong influence over whether an RFID circuit operating as a reader and a tag performs properly. FIG. 1 is a simplified schematic of a known RFID circuit 100. RFID circuit 100 includes a single antenna 110, an IC 120, and an antenna interface 125, which includes a set of external components 130. Antenna interface 125 lies between antenna 110 and IC 120 and is generally identified by the bracket 125 shown in FIG. 1. IC 120 is configured to operate as both a reader and a tag in combination with antenna 110 and antenna interface 125. IC 120 includes first and second pins 120a and 120b for connection with the antenna interface and the antenna.
The particular external components of the set of external components, and the particular configuration of the external components provides that numerous of the foregoing described design objective for RFID, and particularly NFC, may not be met by RFID circuit 100. Specifically, the set of external components includes a first capacitor 130a in series with a second capacitor 130b where the second capacitor is on the antenna side of the first capacitor and is parallel with the antenna. More specifically, the first capacitor is coupled to first pin 120a of IC 120 and the second capacitor is coupled to second pin 120b of IC 120. The set of external components 130 also includes a set of resistors 130c, which is coupled between the antenna and the second capacitor and is coupled to the second pin 120b of IC 120.
The external components and the configuration of the external components provides, for example, that the NFC Forum Specifications for NFC cannot be met at relatively low operating supply voltage (e.g., less than 3 volts).
Further, due to the configuration of the external components, the antenna and antenna interface of RFID circuit 100 cannot be resonant at the NFC carrier frequency when IC 120 acts as a tag, with a relatively high port impedance, if it is resonant at that frequency when IC 120 acts as a reader, with a relatively low port impedance. Not being able to maintain the resonant frequency provides that the RFID circuit may be mis-tuned in a tag mode which increases the sensitivity to interfering signals.
Another deficiency of RFID circuit 100 is that the antenna interface and antenna form a series resonant network at the antenna port. Consequently, when RFID 100 operates in reader mode, the drive current of IC 120, is a maximum when the combination of the antenna and antenna interface, hereinafter referred to as the antenna network, is precisely tuned and is then proportional to the operating Q. The network is precisely tuned and the operating Q is a maximum when no tag is present. The drive current of IC 120 is therefore substantially a maximum if no tag is present for being read, and reduces if a tag is present for the reader to read. With the drive current of the RFID circuit at substantially a maximum if no tag is present, battery power of a mobile device, which includes the RFID circuit, will be used at a relatively high rate for the majority of operation time of the mobile device because for a majority of the operation time a tag is not present for being read. Furthermore, the resistors, 130c, of the antenna interface of RFID circuit 100, which serve to reduce the maximum Q of the antenna interface and antenna and thereby reduce the maximum drive current, also reduce the sensitivity when RFID circuit 100 acts as a tag.